1. Field of the Invention
The present invention relates to a digital-controlled vector signal modulator, and more particularly, to a vector signal modulator directly controlled by digital signal without digital-to-analog signal conversion.
2. Description of the Prior Art
Electronic systems, such as communication systems and test instruments, use vector signal modulators to generate vector signals that meet the amplitude and phase requirement. In a vector signal modulator, a signal is separated to two signals with different phase degree, i.e., the in-phase (I) and quadrature (Q) signals, first. After then, the amplitudes of the in-phase (I) and quadrature (Q) are modulated, respectively, and finally combined together to generate the a vector signal which amplitude and phase both meet requirement. For instance, when the I and Q channels (i.e. signal paths) of the modulator are calibrated to be equal in gain responses, a 45° degree vector signal is generated. °.
Conventional vector modulators utilize variable gain amplifiers (VGAs) to adjust the in-phase and the quadrature signals. However, these VGAs use analog signals to control the gain of the VGAs, and therefore digital to analog converters (DACs) are required. The need of DAC complicates the design of a vector signal modulator and increases production cost.